1000 odorant receptor (OR) genes are expressed in olfactory sensory neurons (OSNs) of the mouse. The expressed OR determines the profile of odorant responsiveness of the OSN, and directs its axon to a specific glomerulus in the olfactory bulb. A central process in the functional organization of the olfactory system is thus the 'choice' of one OR gene per cell, but the mechanisms remain enigmatic. Recent reports show that expression of a functional OR underlies the one neuron-one receptor rule: OSNs that transcribe an OR locus that cannot encode a functional protein frequently choose a second OR gene for expression. Direct or indirect models for OR-mediated exclusive expression of one OR can be invoked. My focus is on the SR1 OR gene. The experimental advantage is that it is expressed at an unusually high frequency and in an anatomically defined structure, the septal organ. In preliminary studies I have demonstrated that a knockout mutation in the SR1 gene has a similar phenotype as reported for 'conventional' OR genes. I have thus validated the premise that SR1 can serve as a model for OR gene choice in general. Specific Aim 1: Specificity, stability and extinction of expression from the SR1 gene locus. I will use gene-targeting in combination with lineage tracing to determine the fate of OR expression in OSNs that do or do not express a functional SR1 coding sequence. In OSNs that are permanently marked for the expression of a functional or non-functional SR1 allele, I will examine the probability of choice, the ability to switch expression to another OR genes, and the extinction of expression from a non-functional OR locus. Specific Aim 2: Is SR1 gene choice limited to a time window during neuronal differentiation? I will develop a conditional knock-out design to eliminate SR1 protein expression from the SR1 locus at two different stages during OSN differentiation. The coding region of the OR gene SR1 will be replaced with GFP as a result of Cre-loxP-mediated recombination in OSNs. I will examine if mature OSNs can re-execute OR gene choice after I engineered the loss of OR expression. Taken together, the proposed experiments will help to elucidate the mechanisms whereby one OSN expresses on OR. [unreadable] [unreadable] [unreadable]